The invention relates to an electro-optic modulator of the kind which may be arranged to modulate a light beam in response to electrical bias. More particularly, the invention relates to an electro-optic modulator in the form of a Fabry-Perot interferometer or etalon having intra-cavity liquid crystal material.
Fabry-Perot etalons containing liquid crystal material are known. They are described in the following references R.sub.1 to R.sub.5 :
R.sub.1 : Offenlegungsschrift DE No. 31,48,427 Al, PA1 R.sub.2 : Offenlegungsschrift DE No. 34,25,584 Al, PA1 R.sub.3 : European patent application No. 0,135,003, PA1 R.sub.4 : Cheung, Durbin and Shen, Optics Letters, Vol. 8, No. 1, January 1983, pages 39-41, and PA1 R.sub.5 : Khoo, Normandin and SO, J. Appl. Phys. 53(11), November 1982, pages 7599-7601. PA1 (1) providing a Fabry-Perot etalon containing intra-cavity liquid crystal material and bias electrodes arranged to bias the material, PA1 (2) passing light through the etalon cavity, the light having an intensity below that at which optical bistability would occur in the liquid crystal material, PA1 (3) varying electrtode bias voltage between two values both of which are above an etalon threshold voltage above which the liquid crystal material refractive index is sensitive to electric bias.
R.sub.1 describes a Fabry-Perot etalon comprising two spaced apart semireflective mirrors with a liquid crystal material sandwiched between them. The mirrors or electrode layers applied to them provide a means for applying a bias voltage across the liquid crystal material. Altering the applied bias changes the refractive index of the liquid crystal material, which in turn changes the optical path length in the Fabry-Perot etalon and alters the etalon transmission wavelength. R.sub.1 is however entirely silent regarding the type and chemical composition of the liquid crystal material to be employed, and also regarding the bias voltage to be applied to this material and its molecular alignment relative to the mirrors or cell walls. In the related field of nematic liquid crystal display devices, bias voltages are normally employed which switch the material from below to above its Freedricks transition. Below this transition, the refractive index of a nematic liquid crystal material is substantially independent of applied voltage. Immediately above the Freedricks transition voltage V.sub.F the refractive index is extemely sensitive to applied voltage, but the rate of change of refractive index as a function of applied voltage reduces with increasing voltage. A typical nematic material cell has a V.sub.F of 1 volt and is switched between 0 and 2 volts.
R.sub.2 describes a liquid crystal cell in which the liquid crystal phase is the so-called blue phase (BP), of which there are two forms, BP1 and BP2. The blue phase exhibits practically no birefringence and produces selective reflection by virtue of the cholestertic phase; ie it behaves in an optically isotropic manner and reflects in a narrow wavelength range. R.sub.2 refers to earlier work on liquid crystal cells, in which the time required to switch the molecular orientation and hence change the cell`s optical properties is about 10 milliseconds or more. This switching time is too great for the purposes of processing video information with associated high data rates. The invention disclosed in R.sub.2 relates to obtaining optical anisotropy in blue phase liquid crystal material, which is normally optically isotropic. However, in FIG. 3 and page 10 lines 18 onwards, R.sub.3 discloses a phase change from BP1 to a cholesteric phase at a temperature of 44.degree. C. and an applied voltage of over 50 volts. This is associated with a substantial change or discontinuity in the optical anisotropy .delta. n/n from virtually zero to an appreciable positive value. As set out in R.sub.2, page 7, lines 1-15, the switching time associated with this preservation of blue phase optical anisotropy is at most 1 millisecond. This compares with 10 milliseconds for previous devices as has been said, and with 0.1 second for the reaction time of what R.sub.2 refers to as the known colour displacement effect. However, the operating temperature at which the change from PB1 to cholesteric occurs is fairly critical. It occurs in one material at 44.degree. C., whereas at 44.5.degree. C. and 48.4.degree. C. respectively the BP2 and isotropic phases are observed. At page 9, lines 22-27, R.sub.2 mentions a different liquid crystal material in which the transitions cholesteric-BP1-BP2-isotropic occur in the temperature interval 26.degree. C.-28.degree. C.
R.sub.2 accordingly describes a liquid crystal which requires temperature control above ambient temperature to a stability of less than 1.degree. C. In this connection it is noted that the specific embodiment of R.sub.2 incorporates a heater. Furthermore, R.sub.2 requires use of a voltage of about 50 volts, ten or more times that employed in conventional integrated circuit technology. The need for a heater increases power consumption requirements.
R.sub.3 describes a Fabry-Perot etalon containing intra-cavity liquid crystal material such as cyanobiphenyl or phenylcyclohexane. The etalon is arranged between two pairs of parallel electrodes, one pair being perpendicular to the other. The optical path length in the etalon cavity is modulated by varying the electric field between the electrodes. In this way coherent radiation may be distinguished from incoherent radiation. R.sub.3 is however silient regarding the frequency of modulation, ie there is no information regarding the operating speed of the device.
R.sub.4 discloses a liquid crystal-filled Fabry-Perot etalon in which optical bistability is demonstrated as a function of varying light intensity, the etalon being placed in a magnetic field. The etalon contains a liquid crystal film 83 microns thick. There is no disclosure of electric field bias being applied to the liquid crystal, nor of switching speed as a function of electric field.
R.sub.5 discloses optical bistability in a liquid crystal cell comprising two glass plates with an intervening layer of liquid crystal material 50 microns thick. The cell is located between two semi-reflecting mirrors forming a Fabry-Perot etalon. As in R.sub.4, there is no disclosure of electric field bias applied to the liquid crystal.
To summarise, Fabry Perot etalons filled with liquid crystal material or containing cells filled with liquid material are known in the prior art. However, they are either employed in applications for which response speed as a function of electrical bias is not important, (R.sub.1, R.sub.3, R.sub.4 and R.sub.5), or alternatively considerable difficulty is experienced in achieving rapid response. In R.sub.2 in particular, temperature control of the liquid crystal material to less than 1.degree. C. is required, together with switching voltages of 50 V or more, for the purposes of achieving switching times in the order of one millisecond or less.
Generally speaking, and as evidenced by the prior art cited in R.sub.2, electro-optic effects observed in nematic liquid crystal materials are too slow (10-200 milliseconds) for use in optical signal processing. Simple nematic devices such as the Freedricks cell or twisted nematic structure are not only comparatively slow, but are slwo somewhat insensitive to applied voltage. They require re-orientation of the nematic director by up to 90.degree. to achieve full contrast switching. The Freedricks cell exhibits greatest optical sensitivity as a function of applied electrical bias at the Freedricks transition voltage, ie the bias voltage above which the refractive index becomes sensitive to electrical bias. Conventional nematic liquid crystal displays switch between first and second bias voltages above and below the Freedricks transition respectively. Displays are however far less critical as regards operating speed requirements than optical modulators.